Orthogonal frequency-division multiplexing (OFDM) is widely used in contemporary radio frequency wireless communication systems because of its spectrum efficiency and outstanding capability in support of multi-user applications. Efforts are being made to bring OFDM into optical communication systems for long-haul applications. However, those efforts have not met with success because OFDM does not perform as well as other, single-carrier solutions in optical transport systems.
More specifically, OFDM can be described as a double-sideband (DSB) modulation scheme, which can be severely distorted by chromatic dispersion (CD) over longer distances. When symbols are transmitted over longer distances, fading caused by CD can result in inter-symbol interference (ISI); that is, chromatic dispersion causes the light pulses that convey digital information to spread, making it more difficult to distinguish the pulses from one another. For a single-carrier system, power fading can greatly reduce the bandwidth of a fiber optic cable, especially for long-haul optical fibers. The impact of CD can be mitigated by optical coherent detection with much higher cost and power consumption. Because of that, optical coherent detection is only deployed in long-haul application scenarios where cost and power consumption are not of concern.
Direct-detected OFDM (DD-OFDM) is receiving attention for shorter range applications, mostly due to its relatively simple structure and low cost. DD-OFDM provides a good trade-off between optical system capacity and simplicity, and is a promising method for metropolitan applications.
CD-induced power fading is also a problem for DD-OFDM. However, as opposed to a single-carrier system, DD-OFDM can assign information bits to sub-channels depending on the frequencies at which power fading is observed. For example, a high-modulation format can be applied in non-fading frequencies, and the use of fading frequencies to carry information bits can be avoided. This technique is an example of the techniques known as “water-filling.” However, the use of water-filling by itself is not the solution to power fading in DD-OFDM systems because it reduces transmission capacity, especially as the number of fading points that are to be avoided increase over long-haul optical fibers.